The present disclosure relates to methods and apparatus for providing signal, voltage and ground lines on any layer of a printed circuit board (PCB), ceramic circuit, hybrid, or similar substrate, where those lines have non-constant width over their respective lengths, and where those non-constant width lines have substantially constant impedance.
The electronics industry has used printed circuit boards for many years. A printed circuit board typically includes an insulating substrate having various electrically conductive lines, sometimes called traces. Electrical components reside on the printed circuit board and interconnect by the conductive lines. Until recently, the operating frequencies of circuits formed by the electrical components on the printed circuit boards did not require tight control of the impedance characteristics of the conductive lines. Even if the circuits had required the control, technology for the mass production of low-cost, high-precision printed circuit boards did not exist.
More recently, the operating frequencies of circuits disposed on printed circuit boards have increased dramatically. The successful operation of such circuits often requires interconnection of the electrical components through electrical pathways having particularly specified impedances. With respect to conventional printed circuit board manufacturing processes, generally only one particular width for a conductive line, or trace, provides the specified impedance. In other words, for a given impedance, for example 50 ohms, only one line width will provide the particular impedance. This line width may differ on different layers of a printed circuit board due to factors such as the thickness of the conductive material, typically metal, on different layers, and the different dielectric materials that may surround conductors on different levels of a multi-layer printed circuit board.
Situations often arise in which an electrical pathway, or line, on a printed circuit board must have a width that varies over its length. For example, a line may have a first width where it extends outward from a solder ball pad, and may then narrow to a second width to escape from a dense Ball Grid Array (BGA) package footprint. Similarly, such a line may widen to a third width to traverse a relatively long distance over the printed circuit board, and may narrow again to pass between a pair of vias in the printed circuit board. Those familiar with the layout of conductive lines on printed circuit boards will understand that many circumstances exist where a single conductive line will have a width that varies along the length of its route.
Variations in the width of a conductive line may result in changes in its impedance characteristics, which may consequently result in degradation of the signal integrity as it travels along the line. A need exists for structures, methods and apparatus for forming the structures, formed in and on low-cost printed circuit boards and similar substrates, which have both variable line width and substantially constant impedance.